The present invention relates to a new electrolyte composition for use in secondary electrochemical cells that employ metal sulfides in the positive electrode and alkali metals in the negative electrode. The development has particular application in a cell that employs FeS as the positive electrode reactant and lithium as the negative electrode reactant. The lithium may be in elemental or alloy form.
A substantial amount of work has been done in the development of high-temperature, secondary electrochemical cells. Positive electrodes in these cells have included chalcogens such as sulfur, oxygen, selenium, or tellurium, and their transition metal chalcogenides as positive electrode materials. The sulfides of iron, cobalt, nickel, and copper are of current interest. Alkali metals and alkaline earth metals such as lithium, sodium, potassium, calcium, magnesium, and alloys of these materials are contemplated as negative electrode reactants. Alloys of these materials such as lithium-aluminum, lithium-silicon, lithium-magnesium, calcium-magnesium, calcium-aluminum, calcium-silicon, and magnesium-aluminum have been investigated to maintain the negative electrode in solid form and improve retention of the negative electrode reactant at high cell-operating temperatures.
In high-temperature secondary cells of this type, current flow between the electrodes is transmitted by molten electrolytic salt. Particularly useful salts include compositions of the alkali metal halides and the alkaline earth metal halides ordinarily incorporating a salt of the negative electrode reactant, e.g. lithium. Eutectic compositions of two or more salts ordinarily are used to minimize melting and cell-operating temperatures. One particularly useful salt composition is the eutectic composition of LiCl-KCl including 58.2 mole percent LiCl and 41.8 mole percent KCl with a melting temperature of 352.degree. C.
The sulfides of iron have received considerable attention as candidates for the positive electrode materials. One of the more formidable difficulties that has impeded the development of cells using these materials, particularly FeS has been the formation of the phrase termed "J phase" (LiK.sub.6 Fe.sub.24 S.sub.26 Cl). The formation of J phase within the positive electrode has been associated with the reluctance of the cells to discharge completely or to fully recharge. This results in low utilization of the FeS positive electrode reactant material and an overall decrease in cell performance.
A number of attempts have been made to suppress formation of the J phase and thereby improve cell performance. One quite successful method has been the addition of sulfides of copper e.g. Cu.sub.2 S to the FeS in the formation of the positive electrode material. Cells with such positive electrode compositions have performed very well for long periods of time. However, after extended cycling, copper metal has been found to deposit within the electrode separators causing electrical shorts and a diminution of cell life.